The present invention generally relates to automatic calibrating or normalizing circuits and, more particularly, to an automatic zeroing circuit that eliminates the effect of dark currents encountered in photometric measuring equipment or similar effects experienced in the electrical output characteristics of transducers other than of the photometric type and detector circuits associated therewith.
A typical apparatus that converts an input, which is sensed by a transducer, to a quantified useful output, either in the form of a visual display or a control signal to a further apparatus, will usually include a transducer, a pre-amplifier detector circuit, a measuring circuit, and an output circuit. Although many types of transducers are known, such as photosensitive detectors, strain gauges, and the like that will produce an electrical signal in response to a sensed input, such as light, mechanical strain, or the like, the present application will present an example of the invention as employed in a colorimeter apparatus. However, it will be understood that the principles of the automatic zeroing circuit of the invention may be similarly employed in other similar detector and measuring systems.
The pre-amplifier detector circuit will amplify the electrical signal of the transducer and will provide an amplified signal output that is electrically measured, for example, in a differential amplifier, in a dual slope integrator, or other electrical measuring equipment. The measured output of such measuring circuitry is provided via the output circuit for display, automatic control, or like purposes. Calibration and other conventional circuits also may be included in such electrical apparatus.
A problem encountered with such detector and measuring systems is that the transducers in their respective circuits usually will produce an electrical signal, such as a leakage current, even when no measurable input is provided thereto. For example, a phototransistor type transducer will have a dark current therethrough even though no light is impinging thereon. Since the measuring portions of such systems are often of the comparator type, they are sensitive to the relatively steady-state or DC errors encountered due, for example, to such dark currents. Moreover, simple calibration of the measuring portion is unsatisfactory to correct for such errors, for leakage current usually is not fixed for a given transducer; rather, the leakage current, such as the dark current of a typical phototransistor, for example, will actually vary as a function of temperature and possibly of other variable parameters, such as aging, etc.
One non-limiting exemplary type of apparatus with which the automatic zeroing circuit of the invention may be employed is disclosed in U.S. patent application Ser. No. 499,479, filed Aug. 22, 1974, now U.S. Pat. No. 3,986,777, for "Tristimulus Colorimeter and Method of Using the Same for Fabrication of Artificial Teeth or the Like". This U.S. patent application is assigned to the same assignee as the present application. To the extent that it may be necessary to facilitate understanding of the present invention, the disclosure of the referenced patent application is incorporated herein.
In such colorimeter, the red, blue and green components of light reflected from an illuminated sample are sequentially and cyclically detected by a photosensitive diode transducer, and the electrical output of that transducer, which has operating electric potentials provided thereto, is amplified in a pre-amplifier detector circuit. A multiplex control responsive to the particular color of light being detected by the transducer at any given time, as determined by the particular position of a rotating color filter wheel, selectively operatively couples respective red, blue and green calibration circuits in the colorimeter and after calibration the respective amplified signals from the pre-amplifier detector circuit are measured in a dual slope integrator circuit type of comparator with respect to a reference electrical potential provided from a zener diode regulated power supply. The multiplex control, moreover, selectively, sequentially, and cyclically controls the delivery of the measured output signals from the dual slope integrator measuring circuit as digital signals to respective output circuits for the red, blue and green colors, which in turn supply the digital signals to respective displays for displaying in digital form the values of the respective color lights detected by the transducer.